Pharmacological analysis of the neurotransmitter mechanisms regulating phenylethanolamine N-methyltransferase in the adrenal gland

The i.p. administration of reserpine daily for 4 days to rats brought about an increase of adrenal phenylethanolamine N-methyltransferase (PNMT) activity. However, the combination of the systemic administration of p-chlorophenylalanine (PCPA) and reserpine for 3 days produced an earlier increase in this adrenal enzyme. The effect was reduced significantly in the denervated gland. Prior administration of 5,7-dihydroxytryptamine (DHT) i.c.v. to rats greatly potentiated the inducing effect of reserpine. On the other hand, the depletion of catecholamines by giving rats alpha-methyl-p-tyrosine (AMPT) i.p. or 6-hydroxydopamine (6-OHDA) i.c.v. did not alter the action of reserpine on adrenal PNMT. PCPA, DHT, AMPT and 6-OHDA did not have any effect by themselves on adrenal PNMT, but the combination of PCPA and AMPT, each given i.p., caused increased adrenal PNMT activity. The administration of dopamine agonists, a treatment that increases adrenal TH, did not modify adrenal PNMT. We conclude that the induction of PNMT by reserpine involves depletion of catecholamines and serotonin, the depletion of serotonin having the more powerful effect. A monoaminergic (serotonergic) inhibitory pathway is involved in the central regulation of adrenal PNMT activity.     

On the role of central nervous system catecholamines and 5-hydroxytryptamine in the nialamide-induced behavioural syndrome

1. Intraperitoneal administration of nialamide, 200 mg/kg, to mice elicited a pronounced increase in motor activity and rectal temperature concomitant with a gradual increase in the concentrations of 5-hydroxytryptamine (5-HT), noradrenaline (NA) and dopamine in the brain.2. In mice treated with L-tryptophan, 300 mg/kg, 1 h before nialamide, the increase in motor activity appeared earlier than after nialamide alone, and the hyperthermia was more pronounced. The increase in 5-HT concentrations in the brain was more pronounced in these animals, whereas the concentrations of NA and dopamine were of the same magnitude as after nialamide alone.3. Treatment with p-chlorophenylalanine methylester-HCl (PCPA), 400 mg/kg, 24 h before nialamide partially antagonized the increase in motor activity and the accumulation of NA and dopamine was not significantly different from that observed after nialamide alone.4. Treatment with PCPA, 800 mg/kg, 72, 48 and 24 h before nialamide, completely antagonized the increase in motor activity and rectal temperature. The accumulation of brain 5-HT was greatly depressed in these animals. The concentrations of dopamine 1, 3 and 6 h and the concentration of NA 6 h after the nialamide injection were significantly lower in the mice given PCPA 800 mg/kg x 3, than in the mice given nialamide alone.5. Administration of DL-5-hydroxytryptophan, 30 mg/kg, 1 h after the nialamide injection, to mice pretreated with PCPA, 800 mg/kg x 3, restored the increase in motor activity and rectal temperature.6. L-Tryptophan, 300 mg/kg, given 1 h before nialamide to mice pretreated with PCPA, 800 mg/kg x 3, elicited a moderate increase in motor activity and a slight increase in the accumulation of 5-HT in the brain when compared to that after PCPA, 800 mg/kg x 3, and nialamide.7. Administration of alpha-methyltyrosine methylester, 200 mg/kg, 2 h before nialamide partially antagonized the increase in motor activity and completely antagonized the increase in rectal temperature elicited by nialamide alone. The accumulation of brain NA and dopamine was inhibited in these animals.8. It is concluded that the excitation in mice, elicited by nialamide, is mediated largely via brain 5-HT, but that also the brain catecholamines seem to contribute to this effect.     

Role of monoamine oxidase inhibition and monoamine depletion in fenfluramine-induced neurotoxicity and serotonin release

The role of both monoamine synthesis and monoamine oxidase inhibition in mediating the fenfluramine-induced damage to serotonin neurones was examined; as pretreatment agents, both alpha-methyl-para-tyrosine (AMPT) and parachlorophenylalanine (PCPA) were used to deplete dopamine and serotonin, respectively, while clorgyline and deprenyl were used to inhibit monoamine oxidase types A and B. While both AMPT and deprenyl did not alter fenfluramine induced serotonin or 5-hydroxyindoleacetic acid (5-HIAA) depletion in any area, PCPA did partially reduce the serotonin depletion in the hippocampus and hypothalamus. Although pretreatment with clorgyline did not significantly alter fenfluramine-induced serotonin depletion, it did produce a 65% mortality rate in animals treated with both drugs. Both PCPA and clorgyline significantly increased the depletion of striatal 5-HIAA concentration consequent to fenfluramine; however, these drugs also produced a long-term depletion of striatal 5-HIAA when administered alone, therefore, the changes seen after the coadministration with fenfluramine may be viewed as additive. Finally, acute PCPA pretreatment attenuated the rapid rise in 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (homovanillic acid) induced by fenfluramine, and acute clorgyline reversed the drop in serotonin and rise in 5-HIAA induced by fenfluramine. These results indicate that the rapid increase in dopamine activity induced by fenfluramine is partially dependent on serotonin concentration and release and that the mechanism of fenfluramine-induced toxicity is unlike that of the other substituted amphetamines.     

Reductions in pain thresholds and morphine analgesia following intracerebroventricular parachlorophenylalanine

The selective decreases in both basal and analgesic pain thresholds following systemic administration of parachlorophenylalanine (PCPA) has been attributed to the inhibition of tryptophan hydroxylase and subsequent depletion of brain serotonin. These effects only occur at high systemic doses which have other general debilitating effects. The present study examined the relationship between PCPA's nociceptive and serotonin-depleting effects following intracerebroventricular (ICV) administration. The first experiment determined that an ICV dose of 3 mg, but not 1 mg, of PCPA significantly decreased jump thresholds at 0.5, 48 and 120 hr after injection. These effects were not due to osmolarity shifts since hypertonic saline injections failed to alter thresholds. The second experiment demonstrated a time-dependent reduction of morphine (5 mg/kg) analgesia as a function of the interval between ICV PCPA and the systemic morphine injection. PCPA reduced morphine analgesia if it was administered 24 hr prior to the opiate and eliminated morphine analgesia if it was administered 48 hr prior to the opiate. Pretreatment with ICV PCPA either 0.5 or 72 hr prior to the opiate failed to alter morphine analgesia. The third and fourth experiments indicated that hippocampal and spinal levels of either serotonin or 5-hydroxyindoleacetic acid were not significantly affected by ICV PCPA pretreatment. These data indicate that the hyperalgesia and morphine analgesia impairments noted following ICV PCPA do not correspond with changes in serotonin from hippocampal or spinal tissue and such effects are discussed in terms of alternative modes of action.     

Role of Monoamine Oxidase Inhibition and Monoamine Depletion in Fenfluramine‐Induced Neurotoxicity and Serotonin Release

Abstract: The role of both monoamine synthesis and monoamine oxidase inhibition in mediating the fenfluramine‐induced damage to serotonin neurones was examined; as pretreatment agents, both alpha‐methyl‐para‐tyrosine (AMPT) and para‐chlorophenylalanine (PCPA) were used to deplete dopamine and serotonin, respectively, while clorgyline and deprenyl were used to inhibit monoamine oxidase types A and B. While both AMPT and deprenyl did not alter fenfluramine‐induced serotonin or 5‐hydroxyindoleacetic acid (5‐HIAA) depletion in any area, PCPA did partially reduce the serotonin depletion in the hippocampus and hypothalamus. Although pretreatment with clorgyline did not significantly alter fenfluramine‐induced serotonin depletion, it did produce a 65% mortality rate in animals treated with both drugs. Both PCPA and clorgyline significantly increased the depletion of striatal 5‐HIAA concentration consequent to fenfluramine; however, these drugs also produced a long‐term depletion of striatal 5‐HIAA when administered alone, therefore, the changes seen after the coadministration with fenfluramine may be viewed as additive. Finally, acute PCPA pretreatment attenuated the rapid rise in 3,4‐dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (homovanillic acid) induced by fenfluramine, and acute clorgyline reversed the drop in serotonin and rise in 5‐HIAA induced by fenfluramine. These results indicate that the rapid increase in dopamine activity induced by fenfluramine is partially dependent on serotonin concentration and release and that the mechanism of fenfluramine‐induced toxicity is unlike that of the other substituted amphetamines.     

Fluoxetine enantiomers as antagonists of p-chloroamphetamine effects in rats

The dextrorotatory enantiomer of fluoxetine was slightly more potent than the levorotatory enantiomer in antagonizing the depletion of brain serotonin by p-chloroamphetamine in rats. The time course of the depletion of brain serotonin at times out to 24 hr after the injection of p-chloroamphetamine was determined with or without simultaneous administration of one of the fluoxetine enantiomers. The dextrorotatory enantiomer prevented the depletion of brain serotonin at any time after p-chloroamphetamine. The levorotatory enantiomer prevented the initial depletion of brain serotonin at 2 and 4 hr, but by 8 hr brain serotonin concentration was decreased and by 24 hr the depletion of serotonin was almost as great as in rats treated with p-chloroamphetamine alone. The elevation of serum corticosterone that occurred acutely after injection of a low dose of p-chloroamphetamine was significantly antagonized by both enantiomers of fluoxetine, the dextrorotatory enantiomer being slightly more potent. In contrast, the lowering of DOPAC (3,4-dihydroxyphenylacetic acid) concentration in rat brain by p-chloroamphetamine was not antagonized by either enantiomer of fluoxetine, indicating this effect is not secondary to serotonin release by p-chloroamphetamine. The results are consistent with other evidence that both enantiomers of fluoxetine are potent inhibitors of serotonin uptake, the dextrorotatory enantiomer being longer-acting than the levorotatory enantiomer in rats.     

Mediation of footshock sensitivity by serotonergic projection to hippocampus.

The present experiments assessed the effects of changes in serotonergic function on footshock sensitivity, as determined by a quantified version of the flinch-jump assessment method. In Experiment 1, depletion of telencephalic serotonin by PCPA injection, medial forebrain bundle lesion, or septal lesion, produced increases in reactivity which were correlated with reductions in telencephalic serotonin levels. In all cases, this increased sensitivity was reversed by injections of d, 1-5-hydroxytryptophan (5-HTP) which restored telencephalic serotonin levels to normal. This effect of 5-HTP had not previously been demonstrated in septal lesioned animals, and overall levels of reactivity of septal animals to other stimuli were also reduced by 5-HTP. Experiment 2 tested the effects of hippocampal lesion on the 5-HTP effect in animals when serotonin depletion produced by either PCPA or septal lesion. Hippocampal lesion, while not increasing footshock sensitivity further, significantly attenuated the effectiveness of 5-HTP in restoring sensitivity to normal. The results suggest that hippocampus may be an important site of action of serotonin in modulating reactivity to footshock, and that failure of raphe lesions to increase reactivity may be due to failure to adequately deplete hippocampal serotonin.     

Effects of central and peripheral depletion of serotonergic system on carrageenan-induced paw oedema

The role of serotonergic system was investigated on peripheral inflammation induced by intraplantary injection of carrageenan. Para-chlorophenylalanine (PCPA) was administered intracerebroventriculary (50, 100 microg/rat) or intraperitoneally (150 mg/kg, 3 days) and 2 or 24 h later, respectively, inflammation was induced by injection of carrageenan. Paw oedema was decreased significantly in PCPA-treated (100 microg/rat, i.c.v.) rats compared to control groups. Injection of exogenous serotonin (i.c.v.) by dose of 0.70 nmol/10 microl/rat, but not the dose of 0.35 nmol/10 microl/rat, 15 min after induction of inflammation completely reversed the anti-inflammatory effects of PCPA. Myeloperoxidase activity in inflamed paws was reduced significantly in groups received PCPA (either i.c.v. or i.p.) compared to controls. Exogenous serotonin (0.70 nmol/10 microl/rat) reduced inflammatory response when injected (i.c.v.) 30 min before or 30 min after the induction of inflammation. Injection of serotonin at the time of induction of inflammation had no inflammatory/anti-inflammatory effect. These results suggest that serotonin, as a neurotransmitter in central nervous system, may be involved in modulating peripheral inflammation.     

A structural approach into human tryptophan hydroxylase and its implications for the regulation of serotonin biosynthesis

Tryptophan hydroxylase (TPH) catalyzes the 5-hydroxylation of tryptophan, which is the first step in the biosynthesis of indoleamines (serotonin and melatonin). Serotonin functions mainly as a neurotransmitter, whereas melatonin is the principal hormone secreted by the pineal gland. TPH belongs to the family of the aromatic amino acid hydroxylases, including phenylalanine hydroxylase (PAH) and tyrosine hydroxylase (TH), which all have a strict requirement for dioxygen, non-heme iron (II) and tetrahydrobiopterin (BH4). During the last three years there has been a formidable increase in the amount of structural information about PAH and TH, which has provided new insights into the active site structure, the binding of substrates, inhibitors and pterins, as well as on the effect of disease-causing mutations in these hydroxylases. Although structural information about TPH is not yet available, the high sequence homology between the three mammalian hydroxylases, notably at the catalytic domains, and the similarity of the reactions that they catalyze, indicate that they share a similar 3D-structure and a common catalytic mechanism. Thus, we have prepared a model of the structure of TPH based on the crystal structures of TH and PAH. This structural model provides a frame for understanding the specific interactions of TPH with L-tryptophan and substrate analogues, BH4 and cofactor analogues, L-DOPA and catecholamines. The interactions of these ligands with the enzyme are discussed focusing on the physiological and pharmacological regulation of serotonin biosynthesis, notably by tryptophan supplementation therapy and substitution therapy with tetrahydrobiopterin analogues (positive effects), as well as the effect of catecholamines on TPH activity in L-DOPA treated Parkinson's disease patients (enzyme inhibition).     

Acute increases by p-chlorophenylalanine of apomorphine-induced stereotyped behavior in the rat

The stereotyped behavioral syndrome induced in the rat by apomorphine was enhanced by acute systemic administration of PCPA. This effect was dependent on the dose of PCPA and half-maximal at approximately 150 mg/kg, i.p.; it occurred within 30 min, was greatest between 1 and 5 h and had nearly disappeared by 24 h after an acute dose of PCPA. A similar effect was not found at 24 or 48 h following 3 repeated doses of PCPA of 300 mg/kg/day. This effect of PCPA was not reversed by 5-HTP or by high doses of a decarboxylase inhibitor. PCPA alone did not produce stereotyped behavior, although it produced some behavioral excitation in high doses following inhibition of monoamine oxidase. This acute behavioral effect of PCPA to potentiate apomorphine-induced stereotyped responses is unexplained. It does not seem to be due to depletion of 5-HT or to the formation of an amine as an active metabolite. We suggest that PCPA can have behavioral excitatory actions independent of its 5-HT-depleting action.Abbreviations CNS (central nervous system) - Dopa (dihydroxyphenylalanine) - 5-HT (5-hydroxytryptamine, serotonin) - 5-HTP (l-5-hydroxytryptophan) - PCPA ([±]-p-chlorophenylalanine methyl ester·HCl) Supported in part by U.S. Public Health Service (NIMH) Grants MH-16674 and MH-25515, and Research Career Scientist Award MH-74370 (Dr. Baldessarini); and an award from the Scottish Rite Schizophrenia Research Foundation, Supreme Council 33°AA, Scottish Rite, Northern Masonic Jurisdiction of the U.S.A.     

Enhancement of tyrosine hydroxylase activity by Oxotremorine does not affect sleep in the rat

Marked enhancement of tyrosine hydroxylase (TH) activity was observed in the rostral pons of Wistar albino rats, 3 days after a single injection of Oxotremorine. TH activity in other brain areas, and regional levels of norepinephrine, dopamine, serotonin and 5-hydroxyindoleacetic acid were unaffected. Enhancement of TH activity did not affect length or number of REM episodes, or the amount of REM occurring in 24 hr. REM occurrence did not, thus, vary in accordance with activity of the rate-limiting enzyme for catecholamine synthesis. These data suggest that, although REM may reflect neuronal protein synthesis, REM is not part of a mechanism regulating the activity of enzymes in the pontine CA synthesis pathway.     

Reevaluation of the regulation of beta-adrenergic receptor binding by desipramine treatment

Treatment of rats with desipramine (DMI) has been shown to down-regulate beta-adrenergic receptor-stimulated adenylate cyclase and reduce the Bmax of beta-adrenergic receptors in some brain areas. Recent reports have indicated that the down-regulation in the number of beta-adrenergic receptors following DMI treatment does not occur if the serotonin system has been impaired following parachlorophenylalanine (PCPA) or 5,7-dihydroxytryptamine injection. We have previously shown that [3H]dihydroalprenolol ([3H]DHA), the most commonly used radioligand to measure central nervous system beta-adrenergic receptors, labels another site under normal experimental procedures, in addition to the beta-adrenergic receptors. This second site has some pharmacological characteristics of the 5-hydroxytryptamine1A receptor. The depletion of serotonin following PCPA injection was indeed able to prevent the down-regulation of [3H]DHA binding sites after DMI injection. However, PCPA alone increased the density of [3H]DHA binding sites. If the nonlinear, least squares, curve-fitting program LIGAND was allowed to define [3H]DHA nonspecific binding or if the more selective beta-adrenergic receptor radioligand [3H]CGP-1277 was used, the Bmax of beta-adrenergic receptors was not changed after PCPA injection. Importantly, PCPA did not prevent beta-adrenergic receptor down-regulation following DMI treatment. The blockade of 5-hydroxytryptamine2 receptors, via ketanserin administration, during DMI treatment did not change the response of beta-adrenergic receptors. Furthermore, if LIGAND was used to define the nonspecific binding of [3H]DHA, the down-regulation of beta-adrenergic receptors was significant 24 hr after a single DMI injection. The same rapid down-regulation was demonstrated with [3H]CGP-12177. However, if [3H]DHA was used to label beta-adrenergic receptors in the "typical" manner (nonspecific binding defined by 10 microM alprenolol), a decrease in the number of beta-adrenergic receptors was significant only after seven daily DMI injections. These data demonstrate that the use of [3H]DHA to measure beta-adrenergic receptors can be misleading, because changes in its second binding site can conceal the changes occurring in beta-adrenergic receptors. Moreover, these results suggest that a similarity in the time course of action of DMI cannot be used to support the hypothesis that its therapeutic antidepressant action is related to beta-adrenergic receptor down-regulation.     

Serotonin depletion enhances the intracerebroventricularly administered MK-801-induced plasma interleukin-6 levels in mice

To investigate the effect of serotonin depletion on the intracerebroventricularly (i.c.v.) administered MK-801-induced plasma interleukin-6 (IL-6) levels, we pretreated mice with parachlorophenylalanine (PCPA, 300 mg/kg, i.p.) 3 d before an i.c.v. injection of MK-801 (1 microg). The i.c.v. MK-801-induced rise of plasma IL-6 level was markedly enhanced in the PCPA-pretreated mice. However, serotonin depletion by PCPA pretreatment did not affect the i.c.v. MK-801-induced increase in plasma corticosterone level. These results suggest that serotonergic system is involved in the suppressive regulation of MK-801-induced plasma IL-6 level.     

Effect of venlafaxine on pineal melatonin and noradrenaline in the male rat

Studies in vitro indicate that the antidepressant drug, venlafaxine (VEN), inhibits the reuptake of both serotonin (5-hydroxytryptamine, 5-HT) and noradrenaline (NA) but has little activity on other neurotransmitter receptors. There are, however, few studies on the effects of VEN on monoamine neurotransmission in vivo. In the present study we examined the effect of VEN treatment on the melatonin content of the rat pineal gland because the synthesis of melatonin is regulated by the release of NA onto pinealocyte beta-adrenoceptors. Acute treatment with higher doses (15 mg/kg) of VEN significantly increased pineal melatonin and NA but this effect was attenuated by subchronic treatment. These data are consistent with in vitro data suggesting that VEN increases NA neurotransmission at higher doses and that repeated treatment can desensitize pinealocyte beta-adrenoceptors.     

Evidence concerning the involvement of 5-hydroxytryptamine in the locomotor activity produced by amphetamine or tranylcypromine plus L-DOPA.

1 Pretreatment of rats with p-chlorophenylalanine (PCPA; 2 X 200 mg/kg) decreased the concentration of 5-hydroxytryptamine (5-HT) in the brain. It also decreased the locomotor activity produced by tranylcypromine plus L-DOPA administration 24 h after the second dose of PCPA. 2 Pretreatment with p-chloroamphetamine, which produced a similar decrease in brain 5-HT concentrations did not decrease the locomotor response to tranylcypromine and L-DOPA. 3 PCPA pretreatment decreased the rise in the concentration of DOPA and dopamine in the brain following tranylcypromine and L-DOPA, suggesting its effect on the dopamine-induced locomotor activity was the result of this drug diminishing dopamine formation in the brain, probably by inhibiting L-DOPA uptake. 4 The locomotor activity produced by tranylcypromine and L-DOPA was not decreased by pretreatment 6 h earlier with disulfiram (400 mg/kg). This argues against the locomotor activity being due to noradrenergic stimulation. 5 PCPA pretreatment did not alter amphetamine-induced stereotypy or the circling behaviour in unilateral nigro-striatal lesioned rats.     

Possible involvement of beta-adrenergic receptors in the enhancement of nocturnal pineal N-acetyltransferase activity due to parathion administration

The purpose of the present study was to examine the effects of administration of sublethal doses of O,O-diethyl-O-p-nitrophenyl phosphorothioate (parathion) on serum epinephrine (EPI) and norepinephrine (NE), as well as on night-time rat pineal melatonin synthesis, both in the presence and absence of propranolol, a beta-adrenergic receptor antagonist. In the first experiment, two groups of adult albino rats were administered parathion orally (1.08 and 2.17 mg/kg/day; the total received by each animal was 6.5 and 13.0 mg/kg body weight over 6 days); another two groups received corn oil only. Animals were killed at 23:00 and 01:00 h by decapitation. Serum EPI was augmented at 01:00 h, but NE was increased at 01:00 and 23:00 h due to administration of the high dose of parathion (13 mg/kg). In the second experiment, two groups of adult male albino rats were administered parathion orally (13 mg/kg); another two groups received an intraperitoneal injection of propranolol (20 mg/kg body weight, 1 h before the lights were turned off). In addition, two groups were given a saline injection. Four hours after darkness onset, pineal N-acetyltransferase (NAT) activity as well as pineal and serum melatonin levels were measured. Parathion by itself significantly augmented nocturnal pineal NAT activity and serum melatonin levels in otherwise untreated rats; the insecticide was ineffective in reference to this enzyme when it was given in conjunction with the beta-adrenergic receptor antagonist propranolol. The augmentation of NAT activity by parathion also caused significant reduction in pineal serotonin (5-HT); again, this response was blocked by propranolol treatment. The results are consistent with the idea that parathion influences pineal 5-HT metabolism either at the level of the beta-adrenergic receptor or via the sympathetic innervation to the pineal gland.     

Prolonged treatment with glucocorticoid dexamethasone suppresses melatonin production by the chick pineal gland and retina

The chick pineal gland and retina synthesize melatonin in a circadian rhythm with high levels during the night. The rhythmic changes in the hormone production result predominantly from the fluctuation in the activity of serotonin N-acetyltransferase (AA-NAT), a penultimate and key regulatory enzyme in melatonin biosynthesis. The aim of this study was to analyze the effects of an acute and prolonged in vivo treatment with a glucocorticoid dexamethasone (4 mg/kg, ip) on the nocturnal increase in AA-NAT activity in chick pineal gland and retina. In acute experiments, dexamethasone (single dose)-injected chicks were killed after 2 h, while in prolonged experiments the glucocorticoid was given once daily for 7 days and the animals were killed 26-32 h after the last injection. Acute administration of dexamethasone did not affect AA-NAT activity in the chick pineal gland and retina. In the pineal glands and retinas of chicks that were treated with dexamethasone for one week and then killed at the end of the light phase of the 12:12 h light-dark cycle, AA-NAT activity was significantly higher than the enzyme activity found in tissues isolated from the vehicle-treated (control) animals. In addition to that, the nocturnal increase in pineal and, to a lower extent, retinal AA-NAT activity was significantly lower in dexamethasone-treated birds when compared with the respective control groups. It is suggested that prolonged treatment of animals with dexamethasone reduces the amplitude of the rhythmic melatonin production, a phenomenon which may affect chronobiological processes being under control of this hormone.     

Depletion of brain serotonin by chronic administration of p-chloramphetamine orally to rats and dogs.

p-Chloroamphetamine (PCA) given in a single oral dose to rats lowered brain serotonin acutely almost as effectively as after ip injection. Likewise, PCA depleted brain serotonin when mixed with the diet, though the amount of food intake was reduced from that normally consumed due to the anorexic action of PCA. In one study in rats fed diets containing PCA for 125 days, the degree of brain serotonin and 5-hydroxyindoleacetic acid depletion was maximal at about 1–4 weeks and diminished during further drug ingestion by rats. In another study in rats fed diets containing PCA for 90 days, brain serotonin concentration was slightly but significantly reduced 2 weeks after drug administration was stopped. Brain serotonin concentration was decreased in dogs at the end of 90 days of oral drug administration and had not returned to control values 2 weeks after drug was discontinued. These results suggest that long-lasting (presumably neurotoxic) effects on brain serotonin neurons are less likely to occur when PCA is given orally than when it is injected ip.     

Involvement of spinal monoaminergic pathways in antinociception produced by substance P and neurotensin in rodents

The antinociceptive effects of substance P and of neurotensin have been determined in rodents after depletion of serotonin (5-HT) or noradrenaline (NA) in the spinal cord. The antinociceptive effect of substance P, given intraventricularly, in rats and mice was blocked after depletion of 5-HT in the spinal cord with the neurotoxin 5,7-dihydroxytryptamine (5,7-DHT) or with the inhibitor of the synthesis of 5-HT, p-chlorophenylalanine (PCPA), but not after depletion of NA in the spinal cord with the neurotoxin 6-hydroxydopamine (6-OHDA). Conversely, the antinociceptive effect of neurotensin in mice was blocked after lesion of spinal NA pathways with 6-OHDA. When 5-HT spinal pathways of mice were lesioned with 5,7-DHT, neurotensin-induced antinociception was blocked 7 but not 15 days after the lesion. p-Chlorophenylalanine failed to prevent this effect of neurotensin. The results suggest that the antinociceptive effect of substance P depends on the integrity of spinal 5-HT neurones, whereas that of neurotensin depends on spinal NA neurones and, only to a limited extent, on 5-HT neurones. It seems that different descending systems are involved in the antinociception elicited by these two neuropeptides.     

Physiological and behavioral effects of parachlorophenylalanine in the rat

The effect of parachlorophenylalanine (PCPA 300 mg/kg i.p.) on several physiological and behavioral parameters was investigated with telemetric methods in the unrestrained rat. Body temperature did not change with the exception of an immediate and short-lasting decrease after drug injection. Food and water intake were maximally depressed on the day following drug administration, and recovered gradually during the subsequent days, drinking more rapidly than feeding. Click-evoked potentials recorded from the auditory cortex and inferior colliculus maintained their typical waveforms during synchronized and desynchronized sleep indicating that PCPA does not produce a qualitative change of the sleep stages. A short-lasting increase of the potentials was observed after drug injection. PCPA exerted profound changes on motor activity. The activity during the light periods was significantly increased. However, motor behavior was altered more in its temporal pattern than in intensity, especially during the dark periods. The circadian rhythms of feeding, drinking and motor activity were attenuated. Since the time-course of these changes corresponds to that known for serotonin depletion in the brain, serotoninergic neuronal mechanisms may play a major role in the organization of behavioral rhythms.This study was supported by the Swiss National Science Foundation, grant no. 3.693.71, and by the Hartmann-Mueller Foundation. Parachlorophenylalanine was supplied by the courtesy of Pfizer AG, Zürich.